1. Field of the Invention
This invention relates to the preparation and formulation of composites comprised of polyimide polymers and copolymers.
2. Description of Prior Art
Composite materials have been used extensively as structural materials in aerospace and other applications where high strength, lightweight materials capable of withstanding high temperatures are required. Recently, considerable efforts have been made to extend the thermal stability range of these materials, while retaining their good structural strength and without increasing the weight of these materials. Currently, addition polymers such as epoxy resins are used in conjunction with fibers or fabrics to provide essentially void-free composite structures of the above class which exhibit good structural properties and are light in weight. These latter structures, however, are limited in their temperatures to about 150.degree.-175.degree. C because of the thermal stability characteristics of epoxy resins.
There are polyimides such as "P13N" from Ciba Geigy Corporation which give very low void content laminates which are useful at temperatures up to 288.degree. C and there is a polyimide known as Kerimid 601 from Rhodia Corporation (a subsidiary of Rhone Poulenc Co.) which can provide void-free laminates which can withstand temperatures of up to 250.degree. C.
Higher temperature laminating resins which cure through addition were apparently unknown prior to the development of the polyimides described in U.S. Pat. No. 3,845,018 assigned to the Hughes Aircraft Company, although there are condensation type polyimides which can be used to produce laminates which withstand temperatures up to 300.degree.-320.degree. C. The condensation resins are limited in their usefulness because the laminates and/or composite materials produced from them exhibit void contents as high as 20-40%. The voids are primarily caused by outgassing which occurs during the condensation mechanism cure.
Applicant Bilow herein, in conjunction with Drs. A. J. Landis and L. J. Miller of Hughes Aircraft Company, developed acetylene-substituted polyimide oligomers which cure through addition rather than condensation in an attempt to solve the void problem discussed above and retain good thermal resistant properties. Polyimide composites with as little as several tenths of 1% voids were obtained even when molding pressures as low as 200 psi were used. These materials are described in U.S. Pat. No. 3,845,018 above.